Optical fiber cables are easy to handle and superior in transmission rate and transmission capacity owing to their lightness and flexibility compared to existing metal cables, therefore they are considered to be the likely winner of the competition for a next generation transmission medium.
However, an optical fiber cable has a drawback in that its elongation at breakage is very small because its material is made of glass. Therefore, when laying an optical fiber cable in a sewerage pipe, the operation needs to be carefully conducted so that the optical fiber cable will not undergo tension.
Studies have been going on about the laying of large diameter optical fiber cables, called a trunk cable or a branch cable and having a outer diameter of about 20 mm, for use in transmission of large amount of information, in the main pipes of sewer pipelines, and also the laying of relatively small diameter optical fiber cables called a drop cable and having an outer diameter of about 5 mm in branch pipes connected to each household from a main pipe.
When laying an optical fiber cable in a sewer pipeline in which a gravitational flow down method is adopted, it is required to lay the optical fiber cable consisting of trunk cables, branch cables, and drop cables in the upper space of inside the sewer main pipe so as not to hinder the sewage flow therein.
Concerning the above described background art, techniques described in JP, B, 3-50483 and JP, A, 8-103012 are known as methods for laying an optical fiber cables in the upper space of inside a sewer main pipe without tensioning the cable.
In these prior arts, a laying robot is introduced in a sewer main pipe while inserting a optical fiber cable raising the optical fiber cable with the robot's arm and pressing it against the upper part of the inner surface of the sewer main pipe and also attaching hooks equipped on the laying robot to the inner surface of the sewer main pipe at an appropriate spacing while hanging the optical fiber cable on the hook.
The method for attaching the hooks in the above described techniques is such that hook mount holes are drilled in the upper part of the inner surface of a sewer main pipe with a drill equipped on the laying robot and after filling an adhesive in a hook mount hole, a hook is forced into the hook mount hole overflowing the adhesive in the hook mount hole to secure the hook in the inner surface of the sewer main pipe.
In this conventional technique, however, an optical fiber cable between hooks may sag under its own weight and therefore the spacing between the hooks needs to be relatively short to prevent a creep rupture due to the sag, and therefore a large amount of time is required for the hook mounting operation.
Moreover, the laying robot is equipped with mechanisms for conducting various operations in a sewer pipeline such as an arm mechanism for pressing the optical fiber cable, a drill mechanism for drilling hook mount holes, an injection mechanism for injecting an adhesive, and a mechanism for forcing the hook into a hook mounting hole. Thus, the robot has a complex structure requiring a high cost. To construct an optical fiber cable network in a sewer pipeline within short time, it is desired that the laying operation consists of a low cost and versatile technique. In this respect, the techniques described in the above publications are not readily adopted.
Furthermore, no consideration so far has been given to the laying of optical fiber cables in branch pipes extending from a sewer main pipe. For this reason, there are occasions that smooth sewage flow is hindered or optical fibers are damaged.